1. Field of the Invention
This invention relates to novel polyorganosiloxanes curable to crosslinked solid materials at ambient temperature upon exposure to oxygen and to novel curable compositions containing same. More specifically, this invention relates to novel RTV polyorganosiloxanes containing one or more 1,3-pentadienylene or 1,4-pentadienylene groups per molecule and gaseous oxygen curing compositions containing same. The novel polyorganosiloxanes and compositions are useful as casting, embedment and encapsulation media and as sealants, caulkings, coatings, including conformal coatings and adhesives.
2. Description of the Prior Art
Several types of RTV (room temperature vulcanizable) silicones are described in the prior art as discussed below:
Siloxanol-silicate type. PA0 Moisture reactive types. PA0 Vinyl-hydrosiloxane type. PA0 Oxygen curable mercaptoalkyl type. PA0 CH.sub.3 (CH.sub.2).sub.3 (CHCH.dbd.CH).sub.2 (CH.sub.2).sub.7 CONHC.sub.4 H.sub.8 --, PA0 CH.sub.3 (CH.sub.2 CH.dbd.CH).sub.3 (CH.sub.2).sub.7 CONHCH(CH.sub.3)(CH.sub.2).sub.3 --, PA0 CH.sub.3 (CH.sub.2).sub.3 (CH.dbd.CH).sub.3 (CH.sub.2).sub.7 CONHC(CH.sub.3).sub.2 (CH.sub.2).sub.3 --, PA0 CH.sub.3 (CH.sub.2).sub.3 (CH.dbd.CH).sub.3 (CH.sub.2).sub.4 CO(CH.sub.2).sub.2 COO(CH.sub.2).sub.3 --, PA0 CH.sub.3 CH.sub.2 (CH.dbd.CH).sub.4 (CH.sub.2).sub.7 COOCH(CH.sub.3)(CH.sub.2).sub.2 --, PA0 CH.sub.3 (CH.sub.2).sub.4 (CH.dbd.CHCH.sub.2).sub.4 (CH.sub.2).sub.2 COOCH.sub.2 CH(OH)CH.sub.2 --, PA0 CH.sub.2 .dbd.CHCH.dbd.CHCOOCH.sub.2 CH.sub.2 C.sub.6 H.sub.4 --, PA0 (CH.sub.3).sub.2 C.dbd.CHCH.dbd.CHC(CH.sub.3).dbd.CHCONHC.sub.6 H.sub.4 --, ##STR3## CH.sub.3 (CH.dbd.CH).sub.4 COSC.sub.6 H.sub.12 --, and the like.
In this earliest RTV silicon system the major polymer component is a linear or lightly branched silicone with silanol terminal groups. Alkyl silicates or alkylpolysilicates, typically tetraethyl orthosilicate or partially hydrolyzed and condensed products therefrom, are used as crosslinking agents. Catalysts are usually tin compounds, e.g. stannous octoate, dibutyltindilaurate. Fillers, color pigments and other ancillaries are also used. The system is usually prepared in two parts which keep the siloxanol polymer and catalyst separated. At point of use, the two parts are mixed and crosslinking begins; thereafter the fluid or plastic working life is limited. Accurately measured proportions and thorough mixing are necessary to produce uniformly cured articles. The need for mixing limits useful compositions to those which are easily stirred and poured and this in turn limits polymer viscosity and level of filler loading. In the early development of this system the role of water was not appreciated. Later, it was established that at least catalytic amounts of water is essential and that unless special steps were taken for its rigorous exclusion prior to use, there was sufficient water absorbed on filler or otherwise fortuitously present to prematurely catalyze the crosslinking reactions.
The next major development in RTV silicones is the one-part system in which a mixture of all components (except water) remains workable in a sealed container until exposed to moisture, usually that present in the normal atmosphere. The major polymer component is a linear or lightly branched silicone having reactive groups (X) that readily undergo hydrolytic polymerization to form siloxane as shown by the following reactions: EQU .tbd.SiX+H.sub.2 O.fwdarw..tbd.SiOH+HX (Rxn 1) EQU .tbd.SiX+.tbd.SiOH.fwdarw..tbd.SiOSi.tbd.+HX (Rxn 2) EQU .tbd.SiX+.tbd.SiOH.fwdarw..tbd.SiOSi.tbd.+H.sub.2 O (Rxn 3)
The reaction groups, .tbd.SiX, may be present in terminal or non-terminal positions or both. A large number of reactive groups are reported in the prior art to be effective. Most commonly used are those containing Si--O--C or Si--N&lt; structures, particularly --OMe, --OEt, ##STR1## --NMe.sub.2, --ONEt.sub.2. The crosslinker component is usually a silane or low molecular weight polysiloxane bearing three or more reactive groups per molecule. A wide variety of catalysts are used, choice depending on the nature of X; metal and amine carboxylate salts are often useful.
Advantages of one-part RTV systems over the earlier two-part systems are that the inconveniences and error incurred in metering and mixing at point of use are eliminated and, since stirring at point of use is not necessary, paste forms can be used and this allows use of higher viscosity polymers and a wider range of fillers and filler levels to achieve improvements in performance properties.
Utility of the moisture reactive type of RTV silicone is limited by the nature of the by-products HX (see Rxn 1, 2) which can be objectionably acidic, alkaline, toxic, corrosive, malodorous or in some other way obnoxious. This limitation has stimulated the search for RTV silicone systems which form only innocuous by-products or even none at all.
In this more recent prior art development, crosslinking is accomplished by hydrosilylation as in the following reaction: EQU .tbd.SiH+CH.sub.2 .dbd.CHSi.tbd..fwdarw..tbd.SiCH.sub.2 CH.sub.2 Si.tbd.(Rxn 4)
The major polymer component is usually a linear or lightly branched silicone with vinyl or other olefinic groups in terminal or non-terminal positions. The crosslinker is usually a low molecular weight siloxane with three or more hydrosiloxane units per molecule. Catalysts are typically platinum compounds which are effective at ppm levels. Compositions of this type which are active at room temperature are two-part systems. One-part systems are made with inhibited catalysts but require elevated temperatures for at least brief periods to activate the catalyst and are therefore not true RTV systems.
A weak point of this kind of crosslinking system is that the platinum catalyst can be poisoned by many substances. A significant advantage is that no obnoxious by-products are formed.
In this more recently developed type of prior art RTV silicone, crosslinking occurs by oxidation of mercaptoalkyl substituents upon contact with molecular oxygen, as in ##STR2## The major polymer component is a linear or lightly branched silicone having mercaptoalkyl substituents, such as 3-mercaptopropyl, on silicon. Crosslinker components (optional) are usually low molecular weight silicones having three or more mercaptoalkyl substituents per molecule. Catalysts are organic compounds of prooxidant metals such as cobalt.
With respect to limitations imposed by by-products, in this system the major by-product is water (Rxn 5) which is considered relatively innocuous and which can be tolerated or, if necessary, removed in many applications. However, under some conditions, side reactions may form small amounts of malodorous and toxic hydrogen sulfide. Furthermore, in contact with sensitive surfaces, such as silver or copper, unreacted mercaptoalkyl groups may have undesirable interactions. Also, compositions of this part which contain disulfide linkages (Rxn 5) on exposure to moisture and air at elevated temperatures can degenerate with formation of corrosive sulfur compounds such as sulfurous and sulfuric acids.